1. Field of the Invention
The present invention relates to a package for sealing a semiconductor element. More particularly, the invention relates to an improvement in a die-attach area of a package for sealing on MOS type semiconductor element.
2. Description of the Prior Art
A metal oxide semiconductor (MOS) is very poor in the voltage resistance and current resistance because the thickness of an insulating film for insulating and protecting MOS integrated circuits is very small.
Accordingly, if static charges are intruded from the outside in the interior of a device including such semiconductor element while the device is being used or transported, breakdown is caused in the insulating film by these static charges, resulting in deterioration of inherent properties of the element or complete breakage of the element, and no normal operation can be expected anymore.
As means for protecting the semiconductor element from breakdown by static charges, there has heretofore been adopted a method in which a common grounding electrode is formed on the element and this common electrode is connected to a grounding terminal.
A conventional package for sealing an MOS type semiconductor element comprises a metal layer 2 composed of gold, which is disposed in the inner bottom portion (the die-attach area) of a substrate 1 composed of a heat resistant insulating substance as shown in FIG. 1. An MOS type semiconductor element 3 is placed on the metal layer 2 and gold silicon eutectic reaction is caused between silicon constituting the element 3 and gold of the metal layer 2 to bond the element 3 tightly to the substrate 1. Respective electrodes of the element 3 are connected to terminals of a lead frame 4 through aluminum fine wires 5. A common grounding electrode of the element 3 is connected to the metal layer 2 through aluminum fine wire 5' for ensuring common grounding, and the metal layer 2 is electrically connected to a grounding terminal of the lead frame 4 through an aluminum fine wire through this connection is not specifically illustrated in the drawing.
After mounting of the semiconductor element the wire bonding, a cap member 6 is bonded on the substrate 1 by a sealing member 7 such as glass or resin, whereby the interior of the package is sealed completely air-tightly.
In case of this conventional package, by the operation of heating the sealing member for attaining air-tight sealing, purple plague is caused in the bonding area between the aluminum fine wire 5' for common grounding and the metal layer 2, and therefore, breakage of the aluminum fine wire or insufficient contact takes place in this bonding area. As a result, the semiconductor device fails to make a normal operation. This is a fatal defect involved in the conventional semiconductor device.
As means for eliminating this defect, there has been proposed a method in which a dummy chip of silicon having an aluminum electrode formed on the top face thereof according to a known technique such as vacuum deposition or spattering is separately mounted on the metal layer 2, the aluminum electrode of this dummy chip is connected to the common grounding electrode of the semiconductor element 3 through an aluminum fine wire and the aluminum electrode is further connected to the grounding terminal of the lead frame through an aluminum fine wire.
According to this method, the aluminum fine wire for common grounding of the semiconductor element 3 is not allowed to have direct contact with gold of the metal layer 2 but is allowed to fall in contact with the aluminum electrode of the dummy chip, and therefore, purple plague is not caused and complete common grounding is possible.
In this method, however, the dummy chip of silicon, which is inherently unnecessary, should be mounted and the troublesome step of fixing this small dummy chip should inevitably be conducted. Further, at the wire bonding step, in order to use a wire bonder, it is necessary to arrange the dummy chip very precisely at a predetermined position. Therefore, this method is not satisfactory in the operation facility and reliability.
As another means for eliminating the above-mentioned defect, there may be considered a method in which a gold fine wire is used instead of the aluminum fine wire for common grounding of the semiconductor element. However, when an ultrasonic wire bonder performing bonding by utilizing frictional heat is used for wire bonding, since gold is soft and slip is caused on the gold fine wire, heat necessary for fusion bonding cannot be obtained and complete bonding cannot be expected. Further, since the gold fine wire is very expensive, there is brought about a disadvantage that the manufacturing cost of the semiconductor device is increased.
It is therefore a primary object of the present invention to provide a package for sealing a metal oxide semiconductor element having a very high reliability, in which occurrence of purple plague by the heating operation at the step of forming air-tight sealing can be completely prevented.
Another object of the present invention is to provide a package for sealing metal oxide semiconductor element, which can be mass-produced at a low cost with good operation facility.